This invention relates to coating printed sheets.
In many applications it is desirable to apply a coating to a printed sheet. For example, a water soluble polymer finish may be applied to a workpiece printed by offset lithography to "dry" the sheet quickly by coating the surface while it is still tacky. This coating avoids the need for powder driers that may be cumbersome or air drying procedures that may be slow. Coatings are also useful for providing a glossy finish that improves the rub-resistance of the workpiece and improves its overall appearance. Finally, adhesive coatings may be applied to printed packaging; for example, heat-set adhesives may be applied to enable attachment of a feature such as the clear plastic bubble of a package used to display the product.
Application of coatings to a workpiece is made difficult by various requirements. For example, the coating should be uniform and its thickness should be controlled. Moreover, the coating should be applied quickly, before its vehicle evaporates causing it to thicken. Finally, it is desirable for the coater to operate "on-line" with the press that prints the workpiece to take full advantage of the fast-drying capability of coatings and generally to simplify the manufacture of printed coated workpieces.
Butler U.S. Pat. No. 4,270,483 discloses an on-line coating apparatus for attachment to a conventional offset lithographic printing press. The apparatus includes a set of rollers (i.e. pick-up roller 14 and application roller 16) to deliver coating material from a reservoir 18 to a blanket roll 108. A metering rod 40 meters the amount of coating transferred to application roller 16.
An on-line coater sold by Norton Burdett Co. of Nashua, N.H. has a single roller driven directly by a D.C. motor. The roller is a gravure cylinder that transfers coating to a blanket cylinder. The coater is attached to a pivoting arm, and the unit can be pivoted away from the press unit when the coater is not in use.
Another on-line coater, sold by IVT Colordry, Inc. of Fairfield, Conn., applies coating from a reservoir pan to a blanket cylinder using a pick-up roller that delivers a metered coating supply to an applicator roller; the applicator roller applies the coating to the blanket cylinder of a press unit.
Kumpf U.S. Pat. No. 3,768,438 discloses a coater in which a fountain roller dips into a coating reservoir and transfers liquid coating material to a feed roller. The feed roller in turn transfers coating material to a coating roller that coats a sheet fed between the coating roller and a format roller.